Microlenses Self-Align to Optical Fibers

Daniel C. McCarthy

The size of microlenses enables them to guide laser beams to or from optical fibers that are hair thin. The difficulty of fabricating such small lenses pales in comparison with the challenge of aligning the lens to a fiber.

The alignment task requires perfectly centering the lens on the fiber output so that their optical axes are collinear. Also, the lens's f number must be appropriate to the distance between the lens and the fiber so that light from the fiber is collimated as it exits the lens.

Light from an optical fiber glued to the back of a photoresist-covered spacer exposes the resist (top) and enables a monomer to form self-aligned microlenses (bottom). Courtesy of the University of California.

Now researchers at the University of California in San Diego have developed a technique that allows the fabrication of lenses that not only are self-aligned to the fiber output, but that are also perfectly collinear with the fiber. The method is simple and low-cost and can produce lenses with diffraction-limited surfaces and f numbers as low as 1.4.

The process starts by coating one side of a glass spacer with RainX, a hydrophobic material used on car windshields. Over this, the group applies a layer of photoresist, while on the opposite side of the spacer, it adheres optical fibers.

White light shone through the fiber selectively exposes the resist to form round footprints at each fiber output. After etching the RainX and resist from the spacer, the array is dipped into a monomer solution. By withdrawing it at a controlled speed, spherical microlenses self-assemble on the hydrophilic domains at the output of each fiber.

And by manipulating process parameters during the withdrawal step, the researchers controlled lens characteristics such as f number and uniformity in an array, said Daniel M. Hartmann, who led the team.

Potential commercial uses include multimode fiber applications, said Hartmann, who has since graduated. Before this can happen, however, the process must be better characterized so that lenses with a given f number can be fabricated reliably, repeatedly and predictably. Also, researchers will need to sample thermally stable polymers for the fabrication process to develop more robust lenses.